High voltage insulator



' July 19, 1960 A. IMHOF HIGH VOLTAGE INSULATOR 2 Sheets-Sheet 1 FiledJune 14, "1954 az zvf July 19, 1960 A. m..-

HIGH VOLTAGE INSULATOR 2 Sheds-Sheet 2 United States Patent ce PatentedJuly 19, 1960 HIGH VOLTAGE INSULATOR Alfred Imhof, Zurich, Switzerland,assignor to Moser- Glaser & p0. A.G., Muttenz, near Basel, Switzerland,a corporation of Switzerland Filed June 14, 1954, Ser. No. 436,656

Claims priority, application Switzerland June 15, 1953 11 Claims. (Cl.174- 30) The present invention relates to high voltage insulatorsconsisting at least partly of synthetic resins and a method formanufacturing same, and more particularly to high voltage insulators forelectrical apparatuses such as transformers, coup-ling condensers, dryrectifiers, and the like.

It is an object of the present invention to overcome the disadvantagesof the high voltage insulators hitherto known in the art which areexplained hereinafter.

An embedding insulation by means of cast resins may prove diflicult,particularly in connection with very high voltages of the electrodes tobe insulated. For instance, the casting of large objects is dilficultowing to the contract-ion of the resin due to hardening and subsequentcooling, and owing to the thermal expansion due to fluctuations of theoperating temperature. If the thermal expansion of the resin exceedsthat of the metal forming the electrodes, the resin is likely to crackand break. Furthermore, the dissipation factor (tan 6) of the knownembedding casting resins is so high at the high operating voltages thatit is sometimes difficult to avoid thermal break-down of the resin.

On the other hand, insulators consisting of a wound web of fabric areeasily damaged. If the webs of fabric are impregnated with a liquid suchas oil they have to be protected against the loss of the liquid by beingplaced inside a container where they may gradually absorb moisture fromthe liquid. However, suitably chosen insulators consisting of wound websof fabric have great advantages such as extremely high breakdownvoltages, a low tan 6 and a high elasticity, so that they are verysuitable for the embedding of field controlling coverings, wire coils,and the like.

The invention overcomes the disadvantages set forth hereinabove in asimple manner by providing a combined high voltage insulating coveringcomprising a dielectric series connection of at least one layer of awound web of insulating fabric and a layer of casting resin enclosingthe same which is at least partly under electrical stress. In thisseries connection of the two dielectrics the embedding resin forms atleast the outermost layer,

and, therefore, a container for the wound dielectric. The connectioncombines essentially the advantages of the two dielectrics without theirdisadvantages.

The casting resin must not be porous, and since 1t has to take up aportion of the total voltage drop, it must have a dielectrically goodquality. Preferably such casting resins are suitable which are obtainedby a poly-reaction such as a polymerization of polyaddition, fromstarting materials which are liquid atroom temperature or increasedtemperatures, particularly from castable materials which harden withoutsplitting off any volatile components. v

According to the invention, the wound insulating webs or bands consistpreferably of a porous dielectric such as paper, impregnated with aninsulating liquid. In order to take full advantage of the highdielectrtic strength of such wound bands, the specific dielectrticresistance thereof should be as large as possible, which is accomphshedby'usmg impregnating liquids having a small dielectrtic constant, suchas mineral oils having a dielectric constant of 2.3 to 2.5, or liquidfiuorinated hydrocarbons, e.g. tri-perfluoro-butyl-t-amine having adielectric con stant of 1.86.

It is well known in the art that laminated paper insulators have adielectric strength which may be much higher than that of a layer ofcompressed gas having the same dimensions, provided that a certain ratioof the pressure to the thickness of the layer is maintained- Pai-'tieularly advantageous is the thus obtainable high shock voltagestrength which, independently of the pressure and the thickness of thelayer, is higher than that of the corresponding gaseous dielectric.

According to an embodiment of the invention, the dielectric strength ofthe laminated paper dielectric is further improved by permeating thepaper web with a "gas or a mixture of gases. e

.In some cases it may be useful to put the gas under such a highpressure that the dielectric strength of the paper insulation permeatedby a compressed gas is higher than that of the same paper insulationpermeated by air under equal pressure and temperature conditions.

Suitable gases are, for instance, gaseous halogens either by themselvesor as components of a gas mixture. However, also other gases may be usedsuch as compressed air, nitrogen, carbon dioxide under normal orelevated pressure, inert gases, or mixtures of air and gases containingchlorine and/or fluorine, furthermore chlorophenyl indane with. orwithout pressure. Gas pressures from 1 to approximately 25 atmospheresmay be used.

The gas permeated paper band may be obtained by placing first theinsulating bandage, then casting and hardening through the syntheticresin layer under vacuum whereupon the apparatus is filled with the .gasor gas mixture. The gas. may be supplied, for instance, by means of avalve arranged in the wall ofthe cast resin, said valve being connectedwith a container containing the gas under pressure; ifdesired, the valvemay be plugged by casting resin after the apparatus has been filled withgas. The filling with compressed gas may be accomplished, for instance,by means of a small rubber plate cast into the resin wall; for fillingthe evacuated apparatus, said rubber plate is perforated by a hollowneedle connected to a container of compressed gas. Later the perforationtogether with the rubber plate can be closed by casting thereinto thesame resin as used for the wall.

For preparing the insulation according to the invention it is ofadvantage to wind the bandage as a first step, then to place the elementin a casting mold, and thereafter to pour the resinous raw material intothe mold. After hardening of the resinous enclosing body, the bandagedinsulation is dried under vacuum; for this purpose, openings may beprovided, if required, in the resinous body. Then the impregnating stepis carried out.

The danger of inflammation which is greater under similar conditionswith insulating liquids than with insulating solids, is largelydiminished by the enclosure consisting of the solid body of syntheticresin. However, in order to eliminate this danger even when the resinenclosure is split open, insulating liquids are used which areincombustible or at least as heat resistant as the cast ing resinitself. Such liquids are, for instance, certain fluoroethers,fluoro-amines, and fluoro-silicones. in order to'prevent bursting of thecasting resin enclosure when overheated during operation, I usepreferably liquid materials having a boiling point as high as possible,preferably exceeding C., such as tri-perfluorohexyl-tamine ortri-perfluorobutyl-t--amine. a

If, for particularly heat-resistant insulators, casting l 3 resins areused which have a high heat resistivity, also the wound insulationconsists preferably of 'a heat resistant material, for instance asbestosband or glass fiber band. This is particularly to be recommended whensuch insulators are provided with wire turns or windings which areheated by the operating current.

In some cases, for instance for high frequency currents, the woundinsulation may'consist also of nonporous thermoplastic materials such aspolystyrene, polyethylene, tetrafiuoroethylene and fiuorochloroethylenepolymers, polyvinyl chloride, polyvinylidene chloride, and others. Thelow heat resistivity of some of these mate rials does not preclude theirapplication since also enclosingcasting resins are available which havea low hardening temperature. Such non-porous foils may be wetted with aliquid dielectric so that they do not enclose any air between the turnsafter the winding thereof, erg. polystyrene may be wetted withmineraloil.

Another insulating material suitable for windings, which offersadvantages in certain cases, is a flexible band rich in mica, consistinge.g. of superimposed layers of mica splittings assembled with thincoatings of insulating binder between each layer.

The combination of insulators according to the invention may be appliedwith advantage also in cases where a field control by conductiveinserts, or an arrangement of wire turns such as those met in primarywindings of voltage transformers, is desired. Conductive inserts, forinstance in the shape of metal cfoils, may be readily wrapped in duringthe winding of the insulating bandage. Normally the ends of thecontrolling insert are arranged inside the wound insulating web (forinstance paper web).

Laminated insulators present on the one hand the advantage that theirelectric strength perpendicularly to the laminated layers is very high;on the other hand, however, they have the disadvantage that the electricstrength in the direction of the laminated layers is much smaller. Atthe border of such capacitive control inserts steep field gradientsoccur, particularly in the direction of the laminated layers. Accordingto the invention this disadvantage is overcome in a simple manner byembedding the ends of the control inserts in the enveloping layer ofcasting resin. 'I his affords also the possibility to provide the endswith a bulge diminishing the field gradient; the bulge is embedded intothe casting resin. r

The insulation according to the invention is furthermore applicable tocondenser leads having one end under oil, the control inserts of whichreach as far as the outer surface lying under oil, and which follow oneanother in radial direction so closely that a high voltage gradient istolerated. Such a fine control may be accomplished according to theinvention by maintaining the control inserts in the correct mutualposition by the wound dielectric and leading them up to the surfacethrough the enveloping layer of casting resin.

In order to reduce the amount of expensive liquid dielectrics used forimpregnating the laminated dielectric, the latter extends advantageouslyin axial direction of the leading-in insulators only as far as requiredby the field strength.

When it is a question of accommodating the wire turns, for instance, involtage or current transformers, the wire turns may be preferablylocated in a free space left between the wound insulation and theenveloping insulation consisting of cast resin. p

Elongated bodies, particularly elongated leading-in insulators may besubject to the difliculty that the resin cover breaks due to theshrinkage at the hardening and cooling thereof. In order to overcomethis drawback, a weak spot advantageously designed as a groove, isprovided according to the invention in a Zone in which the resin coveris subject to a relatively slight electrical stress 50 that the resincover splits in 'said Zone when,

under the influence of contractile forces, the insulator is cooled. Thenthe joint is again cemented in a separate operation by casting resin.

It is also possible to make elongated bodies such as leading ininsulators by casting each of the ends by itself and forming theintermediate part of the leading-in insulator by a metal tube.

The invention will now be described more in detail with reference to theaccompanying drawings, showing, by way of example, some embodiments ofthe present invention. In the drawings:

Fig. lis a sectional elevation of a first embodiment of the invention inform of a current transformer for high voltages;

Fig. 2 is a sectional elevation of a second embodiment of the inventionwith a current transformer of similar design;

Fig. 3 is a sectional elevation of a third embodiment of the inventionshowing a current transformer the pri mary of which is formed by a rod;

Fig. 4 is an elevation, partly in section, of an elongated leading-inrod current transformer with two secondary windings;

Fig. 5 is a section, on an enlarged scale, of the upper end of thetransformer shown in Fig. 4; and

Fig. 6 is a sectional elevation of a modification of the embodimentshown in Fig. 3.

Referring now to the drawings and first to Fig. l, a magnetic annularcore 1 is penetrated by a tube 3 containing the primary windings (notshown) and being provided with a tubular extension or stem 2 whichcontains the current leads (not shown) connected to the primary winding(not shown). The secondary winding 4 surrounds the core 1.

The primary winding (not shown) is wrapped by the insulating paper band5 extending partly also over the stem 2. This part of the insulatingpaper band includes potential control inserts 6 arranged coaxially withthe stem 2 and having lower terminations 7. The insulating paper band 5is arranged inside the insulation 8 consisting of a cast resin andprovided with weather protecting sheaths 9, for instance consisting ofporcelain, and with a cover piece 10. The insulation 8 of cast resin isprovided with a grounded conductive outer layer 11 extendingsubstantially over the lower part of the insulation'8 carrying the core1.

The operation of this device is the same as that of any insulatorprovided with a current transformer.

with an insulating liquid (not shown).

Referring now to Fig. 3 of the drawings, a hollow iron core 101 ispassed through by a primary conductor 102 shaped as a rod and surroundedby the secondary winding 104 which leads to the terminals such as 113.The paper insulation r105 encloses part-of the rod-like primaryconductor 102. The resinous body 108 enclosing the whole structureexcept the ends of the primary conductor 102 is electrically stressedonly at the two conical end portions thereof. The secondary winding 104wound on the magnetic core 101 is in direct contact with the mid portionof the paper insulation 105. An expanding body 114, for instance ahelical spring, is arranged between the one end face of the paperinsulation 105 and the resin body 108. If the magnetic core 101 isprovided with, or replaced by, a grounded part, the apparatus describedhereinbefore may be used as a high voltage leading-in insulator.

Referring now to Figs. 4 and 5 of the drawings, the high voltage currentconductor 15 is closely surrounded over part of its length by animpregnated paper insulation 16 provided with 'condenser control insets17. The paper insulation 16 is enclosed in its middle portion by a metaltube 18, and at the ends thereof by a synthetic resin insulation 19 castthereon. The metal tube 18 is surrounded by ferromagnetic cores 20, 21,carrying, respectively, the secondary windings 22 and 23 of the currenttransformers. A plate 24 serves for attaching the cores and thesecondaries of the transformer to the metal tube 18. Preferably the partof the metal 18 surrounded by the cast resin 19 is provided at theoutside thereof with grooves 25 improving the mechanical connection andthe sealing of the parts 18 and 19.

It has proved to be advantageous to place the paper band under a radialpressure being higher than the pres sure obtainable by the windingoperation. In this way, the dielectric strength of the wound bands isincreased in the direction of the axis. The described and shownembodiments show a cover about the wound bands which is so formed thatthe above condition for the radial pressure is satisfied. A still higherelectrical strength in radial and axial directions is furthermoreobtained by subjecting the impregnating liquid to a pressure exceedingthe atmospheric pressure.

The two measures for increasing the electrical strength may be appliedin combination, thus achieving a further increase of the electricalstrength.

Referring now to Fig. 6 of the drawings showing a rodshaped currenttransformer in section, the ferromagnetic annular core 26 is surroundedby the secondary winding 27 which leads to the terminals such as 28. Thehigh voltage current rod or bar 29 penetrates the core 26 and isinsulated against the same by the wound paper insulation 30 permeated bygas and surrounded by the synthetic resin body 31 enclosing also thecore 26 carrying the secondary winding 27. A valve 32 arranged in thewall of the synthetic resin body 31 serves for conducting the gas to thewound paper insulation 30.

Having thus described the invention and the advantages thereof, it willbe understood that the invention is not to be limited to the detailsherein disclosed, otherwise than set forth in the appended claims.

I claim:

1. A multiple insulation for a high voltage current conductor comprisingan oil impregnated insulating fibrous web surrounding said conductor, ametal sleeve enclosing part of said web, and a sleeve consistingsubstantially of a casting resin, said resin sleeve enclosing said metalsleeve and said insulating web projecting from said metal sleeve andbeing integrally united with said metal sleeve and web.

2. An insulator as claimed in claim 1, including an electricallyconductive winding arranged said wound insulating web.

3. An insulator as claimed in claim 1, including an electricallyconductive winding arranged between said wound insulating web and saidsleeve of casting resin.

4. An insulator as claimed in claim 1, including a cushion of resilientmaterial provided in an area being under a low electrical stress.

5. An insulator as claimed in claim 1, wherein said layer of castingresin encloses only partly said wound insulating web and defines a freeportion thereof, comprising a metal part covering said free portion andengaging said layer of casting resin.

6. An insulator as claimed in claim 5, wherein said metal part is undera radical pressure exceeding the pressure caused by the winding of saidweb.

7. A multiple insulation for a high voltage current conductor comprisingan insulating fibrous web surrounding a rod conductor, said web beingimpregnated with a dielectric, and a sleeve consisting substantially ofa casting resin, said resin sleeve enclosing, and being integrallyunited with, said insulating web and extending beyond said web intointimate contact with said conductor.

8. An insulator as claimed in claim 7 wherein said wound insulating webconsists of gas-filled paper.

9. An insulator as claimed in claim 8 wherein said gasfilled web has adielectric strength higher than the dielectric strength of correspondingair-filled web under equal pressure and temperature conditions.

10. An insulator as claimed in claim 8, wherein the pores of said webare filled with halogen.

11. A method of manufacturing a high voltage insulator comprising thesteps of making a bandaged insulator, casting an insulator consisting ofsynthetic resin around said bandaged insulator, providing a weak zone insaid cast insulator, hardening said cast insulator, allowing said weakzone to break under the influence of contracting stresses, closing saidbreak, and impregnating said bandaged insulator with a fluid insulator.

References Cited in the file of this patent UNITED STATES PATENTS1,759,419 Rump May 20, 1930 2,086,078 Haefely July 6, 1937 2,209,894Scott et al. July 30, 1940 2,272,615 Scott et al. Feb. 10, 19422,287,201 Scott et al. June 23, 1942 2,651,670 Bosworth Sept. 8, 1953FOREIGN PATENTS 888,061 France Apr. 3, 1943 1,032,118 France Mar. 25,1953 OTHER REFERENCES "Epoxy Resin for Enscapulating ElectricalComponents," Electrical Manufacturing, September 1952.

